It is known that soils in general are differently sensitive to the variation of their water content or moisture, which variation is particularly attributable to the natural seasonal cycles of the climate which may promote the origins of phenomena of swelling, loosening and leaching of the bases thereof with resultant change in the equilibrium necessary for ensuring in a manner stable over time a sufficient load-bearing capacity thereof to support a building.
The equilibrium mentioned may change differently over time both owing to natural actions such as variations in the conditions of a layer present in the soil involved, mechanical actions of extreme equipment, climatic variations, etcetera, and owing to anthropic actions such as, for example, carrying out digging operations in soils adjacent to the building, vibrations, losses of fluid in the soil. All these actions, which may act on a specific soil, may accentuate or give rise to, for example, occurrences of subsidence and/or subsequent structural collapse of the buildings located above the soil mentioned, produce physical depressions in the soil and the structures in contact with it such as, for example, vertical structures (walls) or horizontal structures (floors) and which can also become evident in soils with temporary and/or periodic good mechanical properties.
These phenomena of impairment technically define occurrences of differential subsidence and are found partially and locally in soils positioned underneath a foundation of a building, causing the foundation subsequently to collapse with resultant settlement.
It is known in the field that, in order to solve those problems of differential subsidence of foundation soils or construction soils, various techniques are currently used in which, in particular, some tend to transfer the loads of the buildings to lithological planes having a greater load-bearing capacity, with these being more or less deep such as, for example, piles, or expanded bodies, in order to increase the overall profile of the structural foundations, so as to reduce the unit load which the buildings place on the soil.
Other techniques, however, are based on the concept of only pursuing an improvement in the load-bearing capacity of the foundation soil via injections of cement-containing products or chemical formulations, which may also expand, such as, for example, injections of cement at high pressure (jet grouting) or injections of polyurethane foams and the like.
Recently, as described in European Patent Application EP0851064, a possible alternative to the highly pressurized injections of cement-containing fluidized admixtures is the injection with free diffusion of expanding chemical products such as low-pressure polyurethane foams which react and become dispersed in the soil so as to create hardened columns of soil mixed with the resin, at established injection locations which are arranged according to a grid-like three-dimensionally regular distribution, carried out without the use of systems capable of verifying effectively the effects thereof in the course of work directly in the soil. That technique uses indirect monitoring means for the structures outside the soil, by means of laser levels and lifting signalling devices which are positioned so as to be fixed to the walls or floors above the soil undergoing treatment. This teaching indirectly stabilizes the production of the consolidation sought, by means of the criterion which combines the result of the increase in the load-bearing capacity of the soil with the lifting of the structures above which has taken place, as also occurs with the patents EP0941388 and EP1314824.
However, European Patent Application EP1536069 describes a different consolidation method, in which injections of expanding polyurethane resins are carried out in accordance with empirical calculation operations carried out in advance of the injections which, being based on the measurement of the difference in electrical resistance between various locations in the soil, which measurement is obtained by means of instrument sensors which are connected to the injection tubes, allow calculation of the moisture level and, consequently, a definition of the quality and the minimum empirical quantity of expanding resin necessary for that consolidation, but without the provision of subsequent tests during and after the treatment operation.
Another method of consolidating the soil, still by means of injections of expanding polyurethane resins, is described in European Patent Application EP2305894. According to this method, there is carried out an injection of two different types of resin, which are formulated with two different nominal densities having a high and low expansion force, respectively, alternating and in accordance with the injection pressure obtained by the resistance offered by the soil, whilst the resin is in a progressive penetration phase.
The Applicant has observed that this type of injection also does not provide any control, in the course of work, of basic geological parameters for the correct achievement of the operation such as, for example, the porosity of the medium investigated, the degree of saturation, the volumetric water content, etcetera, and therefore does not succeed in obtaining a final control of the correct consolidation.
In fact, however, the experience of the Applicant confirms what has already been verified by other specialized applicants (as, for example, in European Patent EP1536069 in paragraphs [0011] to [0019]), and that is to say that the indirect and empirical evaluation of the consolidation of a soil transferred to the sole measure of the principle of lifting of the subsiding structures or the floor above the point of injection, as also taught in patent EP0851064, affords a significant possibility of error because it does not take into account the geological and geotechnical characteristics of the soil and again only in some cases does that lifting correspond to the definitive consolidation of the foundation soil, the term definitive consolidation of the soil being intended to be understood to be such a state of compaction as to ensure stability and support of the structure above over time, and certainly not quantifiable with measurements of the lifting of the building above.
The Applicant further shares the assertion in paragraph [0019] of the patent EP '069, in which the inventor asserts according to his direct experience that localized geotechnical tests may be of assistance only partially in validating the operation and are difficult to repeat in all the locations of the injections, both from an operating viewpoint and from an economic one.
Similarly in the teaching which can be taken from the patent EP1536069, no consideration is given to monitoring the work in the soil and the relevant effect sought for control of the initial project, efforts being limited to a precautionary calculation of the minimum sufficient quantity of resin to be injected on the basis of a measurement of the electrical resistance of the soil. These methodologies of consolidation according to the prior art seek a solution to the problems of settlement by means of quantitative, mechanical and established actions, directed exclusively at seeking significant increases in load-bearing capacity, by bringing about mainly mechanical compressions in the soil without using direct geological controls, in the course of work, of the effects really induced in the volumes of soil treated, by means for monitoring specific geological parameters of the soil and the problem of settlement. Parameters of this type which merit being monitored are, for example, the porosity of the medium, degree of saturation and volumetric water content, etc. As already indicated in the teaching of EP'069 in paragraph [0018], it is demonstrated that good consolidation of the soil has to above all evaluate the presence (or absence) of holes and cavities, the presence and the quantity of water contained in the soil, as well as naturally the geometry of the foundation plane and the resultant depth of the injections. The fact that they are not considered, and the fact of also not considering a control after the improvement effectively achieved in the soil and optional re-projection with final correction, even partial correction, of the initial project, does not make those techniques sufficiently reliable, or they are intervention techniques which do not allow an assumption of which arrangements are necessary for the injection process so as to achieve the consolidation sought, in a manner which is absolutely considered and effective and therefore durable over time.
The disadvantages linked with the techniques of the prior art are then even more accentuated in soils which are clayey, muddy, peaty or composed of mixed fractions thereof; the Applicant verified in practice that, by modifying the hydraulic conditions of those soils, owing to simple mechanical anthropic compression brought about precisely with the expansion of the resin injected with free diffusion in the soil, it is possible to obtain new concentrations and various distributions of water in those treated soils, which are also obtained only after a few kilograms of injected expanding products. Consequently, by applying the known techniques, it is possible to obtain, even rapidly, a false increase in load-bearing capacity of the soil, verifiable outside the soil with the misleading lifting of the structures, as confirmed by the techniques in the prior art. If, however, after the erroneous consolidation, the same treated volumes of soil become subjected to phenomena of cyclical and seasonal drying and re-expansion, owing to natural climatic variations in the medium and short term, the initial problems of subsidence will occur again, or in part, because they are still dependent on an erroneous distribution of the content of water and holes, a consequence of the resins injected in a standardized manner in the soil, for lack of adequate geological control during the work. It is known that a soil in which water has erroneously been confined under pressure by means of injections of expanding resins offers mechanical resistance values, for example, in the penetrometer vertical test (1D), which sometimes appear to be satisfactory upon immediate observation, but which result in fact in a non-definitive misleading mechanical result, since it is influenced by interstitial over-pressures of water which are erroneously obtained in the soil.
Only afterwards, when the interstitial water content contained in the soil becomes slowly diluted, naturally becoming dispersed according to time and methods mainly as a function of the type of lithology, granulometry and the ambient climatic conditions, will it be possible to establish the recurrence of the original differential occurrences of subsidence. This limit is almost confirmed by all the techniques set out above which become evident, in the criterion validating the consolidation, by the control of the start of the geometric lifting of the treated soil.
The Applicant has recently proposed a methodology for intervention in European Patent Application No. EP1914350, setting out a consolidation method in which there is provided the control of the injections during work by means of the 3D tomography of electrical resistivity of the soil being treated. As published in Engineering Geology 119 (2011) by G. Santarato et al. in the article “Three-dimensional Electrical Resistivity Tomography to control the injection of expanding resins for the treatment and stabilization of foundation soils”, page 18 ff., there has been carried out and developed a method which brings about correct and effective consolidation of the subsided soil, by carrying out expanding injections in a considered manner, owing to the results progressively obtained by the 3D tomography of the electrical resistivity, in which, during work, with the geoelectrical monitoring always being maintained in operation, it becomes necessary to modify, as necessary, the parameters of injection in accordance with the effects sequentially encountered in the soil and in such a manner as to make uniform the chemico-physical characteristics of the subsided volume with those of adjacent volumes of soil which have not subsided and which are taken as a reference.
It is known that, based on the measurement of the electrical resistivity via the application of the formula of Archie, in which, in first approximation, the resistivity of the soil is directly proportional to the following parameters of the soil:rm=aS−np−mrw where:                rm resistivity of the soil,        a, m, n empirical constants,        S degree of saturation,        p porosity of the medium, and        rw resistivity of the fluid in the porous medium,it is possible to determine the porosity of the medium being investigated, the degree of saturation and the volumetric water content (Kalinski and Kelly, 1993), all geological parameters which are of fundamental importance both in terms of the preliminary evaluation of the causes of the subsidence and subsequently of the effects of improvement which follow the consolidation by means of injections of expanding admixtures.        
Although this last teaching solved the problem of geological control, during work, of the effects in the soil caused by the injections of resin, by means of the sequential measurement in quasi-real time of the electrical resistivity, sometimes suffers from the limitation that it is not always possible to conveniently provide for an adjacent volume of soil which has not subsided and which, owing to chemico-physical and structural characteristics, can be taken as a reference in the subsequent stabilization and homogenization process of the subsided volume of soil.